The present invention relates to a printing apparatus and suction recovery control method, and more particularly, to a printing apparatus employing a printhead which performs printing in accordance with an ink-jet printing method, and suction recovery control method.
A printing apparatus, employed in a printer or a printer unit of a copy machine or facsimile apparatus or the like, prints an image based on inputted image data by forming dot patterns on a print medium, e.g., paper, thin plastic sheet, fabric or the like.
Such printing apparatus can be categorized according to printing methods, e.g., ink-jet method, wire dot method, thermal-transfer method, laser-beam method and so on.
Among these printing apparatuses, the type employing ink-jet printing method which performs printing by discharging ink from a printhead to a print medium is advantageous, not only because printing can be performed with high precision at high speed, but because printing can be performed with low noise by virtue of the non-impact method, and color images can easily be printed with multiple colors of ink.
Furthermore, according to a bubble-jet method of the ink-jet printing method, ink is heated to cause film boiling and ink is discharged by pressure of bubbles generated by the film boiling. The bubble-jet method is known to realize high resolution printing and high speed printing even more easily.
An ink-jet printing apparatus, using ink as a recording material for printing, attributes importance to reliability maintenance for the ink discharge function of a printhead, in order to prevent negative influence on printing, caused by ink evaporation or bubble mixture in ink.
More specifically, while an ink-jet printing apparatus is performing printing operation or is not in use, bubbles are gradually generated inside the ink discharge nozzles of the printhead or in the inserted portion of the printhead. This may disable ink discharge (no discharge) or cause discharge failure, disabling normal print operation. In order to eliminate these bubbles, the ink-jet printing apparatus comprises a cap for capping the printhead, and a head recovery unit having a suction pump for sucking ink inside the cap. The ink discharge surface of the printhead is capped at the position where the printhead faces against the cap, and the suction pump sucks bubbles inside the printhead. The suction recovery processing is an important technique for reliability maintenance of the ink-jet printing apparatus.
However, even if a suction condition is determined so as to make full use of the bubble-eliminating capability of the head recovery unit, in reality, volumes of bubbles vary in different suction operations. Therefore, the same bubble-eliminating performance cannot always be achieved. In view of this, in order to maintain excellent bubble-eliminating performance, the conventional ink-jet printing apparatus counts the number of times of discharge, or times the non-printing state period of the apparatus, or executes both, so that the suction is performed while the bubble volume is as constant as possible, and then controls suction operation in accordance with the counted values. More specifically, the timing of suction operation of the printhead is determined based on the point of time whichever earlier: at which a predetermined time has elapsed from an initial point of time, or at which a predetermined amount of printing is completed from the initial point of time.
Japanese Patent Application Laid-Open (KOKAI) No. 6-238914 proposes a method of determining timing of suction operation of a printhead based on the number of times of discharge, non-printing state period of the printer, and temperature of the printhead, taking into account a difference in the bubble generation amount in the head caused by a temperature rise inside the printhead.
However, the reliability maintenance employed in the conventional ink-jet printing apparatus does not consider the number of times of discharge per unit printing area of a printhead (e.g., actual number of times of discharge while an image corresponding to one line is printed on A4-size paper. Hereinafter referred to as a print image duty). Therefore, depending on a print image duty, suction operation is unnecessarily performed, causing to reduce the throughput of the printing apparatus or increase the amount of wasted ink, or causing discharge failure by bubbles before suction operation is performed.
Particularly, the bubble-jet method employs a method of locally heating ink to cause film boiling to generate ink discharge energy. According to this method, the internal temperature of the printhead gradually rises as printing operation is performed, and as a result of the temperature rise, the generation state or growth rate of bubbles changes. In view of this, as described above, the conventional example proposes a method of determining the timing of suction operation of the printhead based on the number of times of discharge, non-printing state period of the printer, and printhead temperature.
However, this method has the following problems.
(1) The temperature rise of a printhead varies for each head.
(2) Temperature detection accuracy of a printhead varies, making accurate control difficult.
(3) The internal temperature of a printhead differs depending on a pattern of a printing image.
Hereinafter, these three problems are described further in detail.
The following facts have been discovered as a result of careful study of causes and mechanism of bubble generation in a printhead which causes discharge failure.
In a case where printing operation is performed by a printhead employing an ink-jet printing method, small bubbles are first generated in the printhead, and then these bubbles are coalesced to grow into large bubbles. After the bubbles are first generated, if ink is not discharged from the printhead, these bubbles melt in the ink and disappear. Therefore, it is considered that the condition for small bubbles to coalesce and grow into large bubbles is to repeat ink discharge from the printhead within a predetermined time period before small bubbles disappear. In other words, if the printhead performs many times of ink discharge within a unit time period, bubbles coalesce and grow before disappearing, thus causing discharge failure.
FIG. 10 shows a relation between the number of lines printed by a printhead before discharge failure occurs, and a print time duty. Note that the print time duty is the number of print dots per unit time (dot/second).
As can be apparent from FIG. 10, as the number of print dots per unit time increases, discharge failure occurs at the smaller number of line, i.e., discharge failure occurs in the earlier stage.
If the internal temperature of the printhead is further taken into consideration, the case where the number of print dots per unit time is large and the internal temperature is low is more likely to cause discharge failure than the case where the number of print dots per unit time is small and the internal temperature is high.
As described above, mere detection of a printhead temperature is not sufficient for determining suction operation timing. Particularly when an ink-jet printing apparatus is in a print stand-by state during print data transfer, growth of bubbles, i.e., occurrence of discharge failure, is more highly correlated with the number of print dots per unit time than the printhead temperature, as is apparent from the aforementioned study. Furthermore, in a case where an ink-jet printing apparatus executes control (hereinafter referred to as temperature rise detection) such that printing is allowed only when the printhead temperature is lower than a predetermined temperature, discharge failure occurs depending on the print time duty rather than the printhead temperature.
In other words, mass-produced ink-jet printheads differ in various printing characteristics. Therefore, even in a case where the same print data is inputted to print the same image, consideration must be given in that there may be a printhead which easily raises temperature and a printhead which does not easily raise temperature. Therefore, despite execution of temperature rise detection, the printhead which does not easily raise temperature is more likely to allow printing than the printhead which easily raises temperature. In other words, the printhead which does not easily raise temperature is more likely to perform printing consecutively or perform a number of times of printing within the unit time than the printhead which easily raises temperature.
Therefore, the printhead which has a low temperature and does not easily raise the temperature, is more likely to cause discharge failure by bubbles in the printhead, than the printhead which has a high temperature and easily raises the temperature. As described above, an ink-jet printing apparatus raises a problem in that the conventional temperature rise detection cannot sufficiently prevent discharge failure because there are cases where discharge failure due to bubbles in the printhead occurs when a printhead has a low temperature rather than a high temperature.
Furthermore, in the conventional method of determining the timing of suction operation of a printhead based on the number of times of discharge, non-printing state period, and printhead temperature, there is a problem because the precision of a head temperature sensor of the ink-jet printing apparatus is not sufficient.
More specifically, even if the printhead temperature (detected temperature) detected by the sensor of the printing apparatus is low, there may be cases that the actual printhead temperature is higher than the detected temperature. In this case, suction operation is not performed, causing discharge failure. On the contrary, even if the printhead temperature (detected temperature) detected by the sensor of the printing apparatus is high, there may be cases that the actual printhead temperature is lower than the detected temperature. In this case, the number of times of suction operation may unnecessarily increase, resulting in a reduced throughput of printing operation and the increased amount of wasted ink.
Furthermore, there may be cases where the timing of suction operation cannot accurately be determined depending on the printing pattern. For instance, as shown in FIG. 11A, in a case of printing a pattern having a high print image duty in the first half of a print medium and a pattern having a low print image duty in the latter half of the print medium, the printhead temperature detected at the end of printing the pattern on the print medium is relatively low compared to a case of printing a pattern having a uniform print image duty on the entire print medium. Because of this, suction operation is not performed, causing discharge failure.
On the contrary, as shown in FIG. 11B, in a case of printing a pattern having a low print image duty in the first half of a print medium and a pattern having a high print image duty in the latter half of the print medium, the printhead temperature detected at the end of printing the pattern on the print medium is relatively high compared to a case of printing a pattern having a uniform print image duty on the entire print medium. Because of this, the number of times of suction operation may unnecessarily increase, resulting in a reduced throughput of printing operation and the increased amount of wasted ink.
The above-described problems may be solved by executing suction recovery operation while printing one page of print medium. However, performing suction recovery operation may cause to change the state of ink in a printhead. Therefore, if suction recovery operation is performed during printing of one page of print medium, tonality of an image printed on the print medium may change. For this reason, it is not preferable to execute suction recovery operation during printing of one page of print medium.
As summarized, it is extremely difficult to execute appropriate printing control according to the printhead temperature of an ink-jet printing apparatus.
The present invention is made in consideration of the above situation, and has as its object to provide an ink-jet printing apparatus and suction recovery control method which can minimize reduction in throughput while maintaining a printhead in the most appropriate condition, and can reduce the amount of wasted ink.
According to one aspect of the present invention, the foregoing object is attained by providing a suction recovery control method of controlling suction recovery operation of a printhead used in an ink-jet printing apparatus, comprising: a first timing step of timing a printing period of predetermined printing operation; a counting step of counting a number of print dots formed by discharging ink from the printhead during the predetermined printing operation; a calculating step of calculating a number of print dots per unit time based on the printing period timed in the first timing step and the number of print dots counted by the counting step; a deciding step of deciding a timing of the suction recovery operation based on the number of print dots per unit time calculated in the calculating step; and a control step of controlling the suction recovery operation of the printhead at the timing decided in the deciding step.
Herein, it is preferable that the deciding step includes: a correction step of correcting the counted number of print dots based on the calculated number of print dots per unit time; a first comparing step of comparing the number of print dots, corrected in the correction step, with a predetermined threshold value each time printing of one page of print medium is completed; and a determining step of determining whether or not to perform suction recovery operation according to a comparison result of the first comparing step.
Furthermore, the aforementioned method further comprises a second timing step of timing a cumulative time period, starting from when power to the ink-jet printing apparatus is first turned on.
It is preferable that the deciding step includes a second comparing step of comparing the cumulative time period with a predetermined threshold value each time the printing of one page of print medium is completed, and in the determining step, the timing of suction recovery operation is determined according to a comparison result of the second comparing step.
Note that the predetermined printing operation corresponds to performing printing of one page of print medium or performing printing of one scan of the printhead.
Furthermore, the predetermined threshold value employed in the first and second comparing steps is obtained by experimentally counting, in advance, a number of print dots formed before the printhead results in ink discharge failure.
Furthermore, it is preferable that in the determining step, the timing of suction recovery operation is a point of time whichever earlier: at which the number of print dots, corrected in the correction step, exceeds the predetermined threshold value in the first comparing step; or at which the cumulative time period exceeds the predetermined threshold value in the second comparing step.
According to another aspect of the present invention, the foregoing object is attained by providing a suction recovery control method of controlling suction recovery operation of a printhead used in an ink-jet printing apparatus, comprising: a counting step of counting a number of print dots, formed by discharging ink from the printhead, every predetermined printing area; a calculating step of calculating a number of print dots per unit printing area based on the number of print dots counted in the counting step and a total number of dots printable in the predetermined printing area; a deciding step of deciding a timing of the suction recovery operation based on the number of print dots per unit printing area calculated in the calculating step; and a control step of controlling the suction recovery operation of the printhead at the timing decided in the deciding step.
It is preferable that the deciding step includes: a correction step of correcting the counted number of print dots based on the calculated number of print dots per unit printing area; a first comparing step of comparing the number of print dots, corrected in the correction step, with a predetermined threshold value each time printing of the predetermined printing area is completed; and a determining step of determining whether or not to perform suction recovery operation according to a comparison result of the first comparing step.
Furthermore, it is preferable that the aforementioned method further comprises a timing step of timing a cumulative time period, starting from when power to the ink-jet printing apparatus is first turned on.
In this case, it is preferable that the deciding step includes a second comparing step of comparing the cumulative time period with a predetermined threshold value each time printing of the predetermined printing area is completed, wherein in the determining step, the timing of the suction recovery operation is determined according to a comparison result of the second comparing step.
Note that the predetermined printing area corresponds to an area printed by one scanning of the printhead.
According to still another aspect of the present invention, the foregoing object is attained by providing a printing apparatus for printing on a print medium by using an ink-jet printhead, comprising: suction recovery means for performing suction recovery operation of the ink-jet printhead; first timing means for timing a printing period of predetermined printing operation; counting means for counting a number of print dots formed by discharging ink from the ink-jet printhead during the predetermined printing operation; calculation means for calculating a number of print dots per unit time based on the printing period timed by the first timing means and the number of print dots counted by the counting means; decision means for deciding a timing of the suction recovery operation based on the number of print dots per unit time calculated by the calculation means; and control means for controlling the suction recovery operation at the timing decided by the decision means.
Furthermore, it is preferable that the aforementioned apparatus further comprises second timing means for timing a cumulative time period, starting from when power to the printing apparatus is first turned on, wherein the decision means decides the timing of the suction recovery operation while further considering the cumulative time period.
According to still another aspect of the present invention, the foregoing object is attained by providing a printing apparatus for printing on a print medium by using an ink-jet printhead, comprising suction recovery means for performing suction recovery operation of the ink-jet printhead; counting means for counting a number of print dots, formed by discharging ink from the printhead, every predetermined printing area; calculation means for calculating a number of print dots per unit printing area based on the number of print dots counted by the counting means and a total number of dots printable in the predetermined printing area; decision means for deciding a timing of the suction recovery operation based on the number of print dots per unit printing area calculated by the calculation means; and control means for controlling the suction recovery operation of the printhead at the timing decided by the decision means.
Furthermore, it is preferable that the aforementioned apparatus further comprises timing means for timing a cumulative time period, starting from when power to the printing apparatus is first turned on, wherein the decision means decides the timing of the suction recovery operation while further considering the cumulative time period.
Furthermore, in a case where the apparatus comprises scanning means for reciprocally scanning the ink-jet printhead, the predetermined printing area corresponds to an area printed by one scanning of the ink-jet printhead.
The printhead mentioned in the foregoing configuration comprises electrothermal transducers for generating heat energy to be applied to ink so as to discharge ink by utilizing the heat energy, and the printhead discharges ink from discharge orifices by utilizing film boiling in the ink, which is generated by heat energy applied by the electrothermal transducers.
According to the present invention described above, the printing period of predetermined printing operation is timed, and the number of print dots formed by discharging ink from a printhead during the predetermined printing operation is counted. Based on the timed printing period and counted number of print dots, the number of print dots per unit time is calculated. Then based on the calculated number of print dots per unit time, timing of suction recovery operation is determined, and suction recovery operation of the printhead is performed according to the determined timing.
Alternatively, the number of print dots formed by discharging ink from a printhead is counted each time printing of a predetermined printing area is completed. Based on the counted number of print dots and a total number of dots printable in the predetermined printing area, the number of print dots per unit printing area is calculated. Then based on the calculated number of print dots per unit printing area, timing of suction recovery operation is determined, and suction recovery operation of the printhead is performed according to the determined timing.
The present invention is particularly advantageous since the timing of suction recovery operation of a printhead can be determined by taking into consideration an influence imposed on the printhead actually used in the printing operation.
Therefore, for instance, even if a temperature characteristic of a printhead varies, suction recovery timing can be accurately determined without being influenced by such variation of characteristics. Accordingly, a printhead can be maintained in the most appropriate condition with the minimum number of times of suction recovery operation, reduction in throughput of printing operation can be minimized, and the amount of wasted ink generated by suction recovery operation an be reduced.
Furthermore, also in a case of employing a printing apparatus having the function of printhead temperature detection, the present invention can prevent a situation where temperature detection precision or print data or a printing pattern influences the temperature detection result, and negatively influences the suction recovery timing of a printhead.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.